When cellular communication systems were first introduced under the Advanced Mobile Phone Service, Inc. (AMPS), separate sections of the available spectrum were allocated to wireline carriers (telephone companies) and non-wireline carriers (other communication providers who were not then involved in telephone communication). Of course, when the AMPS specification was first published, in March of 1981, it was presumed that cellular service providers would be primarily telephone companies, with other radio common carriers (RCC's) making up the balance. Existing RCC's were principally involved in conventional two-way systems, paging, and trunked communication.
The original allocation plan called for a large section of radio frequency (RF) spectrum to be allocated to cellular communication, covering two ranges of frequencies offset by 45 MHz (megahertz). The frequency offset was designed to support full-duplex communication, so that communicating parties could both talk and listen at the same time. This was necessary, of course, so that cellular communication would approximate landline telephone communication as closely as possible.
A range of frequencies from about 870 MHz to around 890 MHz was set aside for "forward" channels. By forward, the drafters of the AMPS spec meant communication occurring in what they termed a forward direction: from base stations to mobile or portable cellular telephones. The frequencies set aside for "reverse" communication (from cellular mobiles or portables back to base site equipment) were offset by 45 MHz as mentioned above, and ranged between about 825 MHz and 845 MHz. These frequency bands were arranged in 666 RF channels, each 30 KHz (kilohertz) apart, much as represented in FIG. 1. Wireline carriers were assigned the lower 333 channels, designated as Band A, while non-wireline carriers were assigned the upper 333 channels. This division of spectrum was conceived largely on the presumption that there would be two competing cellular systems in most markets. As illustrated in the figure, there was reserved spectrum at each end of both sections of allocated spectrum.
In the design of RF receivers, it is common practice to provide filtering in the receiver "front-end" to reject signals that fall outside the frequency band of interest. For a cellular carrier in Band A, for example, it would be appropriate to provide a bandpass filter in the receiver front-end that would pass the Band A channels, while rejecting everything outside that range. Filters meeting these general requirements were easily provided using conventional bandpass filter technology.
Unfortunately, as cellular popularity boomed, the clamor for more communication channels was not met by the foreseen spectrum expansion that would have simply extended Band A and Band B at their upper and lower limits, respectively. Instead, Band A was extended by adding 1 Mhz of spectrum just below the A Band (from 824 to 825 MHz), and another 1.5 MHz just above the B Band (from about 845 to 846.5 MHz). The B Band extension was similarly discontinuous, but was accomplished by adding a single section of RF spectrum, from 846.5 MHz to 849 MHz. Of course, the new frequencies just mentioned are for reverse channels. Similar segments were added, 45 MHz away, for forward channel expansion. The results of this cellular spectrum expansion are illustrated in FIG. 2.
The greatest impact of this segmented expansion plan was felt in the design of filters for A Band receivers. Although the B Band was also segmented, there is only a 1.5 MHz gap between B Band spectrum sections. Consequently, a high-Q ceramic notch filter cascaded with a conventional bandpass filter adequately solves the filtering dilemma, without introducing insurmountable design problems. The A Band situation, however, is somewhat different.
Since the disparate segments of Band A are separated by a 10 MHz gap, it is difficult to cascade bandpass and band reject filters to meet the necessary specifications without meeting interaction problems related to impedance mismatch and phase distortion. Consequently, a need arises for a filter that integrates the desired bandpass and band reject (or notch) characteristics into a single filter, so that problems with filter interaction can be minimized and costs can be reduced.